Study the Evolution of Laishe River Watershed and Channel Nearby the Confluence of Laishe River and Neishe River after Morakot Event

Abstract

Extremely heavy rainfall events often induce large scale of landslides in the catchment Tons of sediment would transport into the river letting the riverbed suddenly raised and changing the original state of the river This study took the junction of Laishe River and Neishe River as well as nearby reach as research subject Our study first segmented the study reach into 7 sub-stretches and named them by the characteristics We analyzed evolution of riverbed elevation changed in each stretch under the disturbance of rainfall events and artificial dredging projects from 2009 to 2018 by annual Digital Elevation Model (DEM) data we collected The results show that the elevation of the river nearby the confluence arose 12 1 meters in average after Morokot event The alluvial plain expanded 3 8 times than before and the aggradation level of main channel elevation increased from downstream to upstream Each stretch demonstrated different elevation changed trend in subsequent years The upstream stretch incised a lot in initial period then the speed of incision decreased The stretches nearby the confluence aggraded in early period whereas degraded after 2014 Between 2013 and 2016 the stretches nearby the confluence were affected by the dredging project The elevation of main stream degraded in the dry season however aggraded by upstream sediment transport after rainy season The slope of the river still changed The balance between transportation and deposition continues to change Through extensive data collection this study records the evolution of the Laishe River and the catchment area after the Morakot event describes and speculates on the process and causes of the river elevation change and provides a reference for future river restoration after severe sediment events

Date of Award	2019
Original language	English
Supervisor	Chyan-Deng Jan (Supervisor)